Recent observations suggest that when, seasonally, the external environment begins to degenerate in relation to free-living larval development, infective hookworm larvae may be physiologically modified so that their subsequent development in the host includes a dormant state lasting several months. Based on this hypothesis, the broad objectives of this investigation are: 1. To develop a model-parasite system in which arrested development of the parasitic larval stages of hookworms can be studied. 2. To examine various factors associated with a degenerating external environment for their ability to induce a potentially dormant state. 3. To experimentally provoke the resumption of larval development in the host, and thereby contribute to the identification of factors which release larvae from the arrested state. To accomplish these objectives: 1. One or more of the several common hookworm species of dogs and cats will be used. 2. Degenerating ecological conditions will be simulated during maintenance of infective larvae in environmental chambers. 3. Infective larvae exposed to these unfavorable conditions and larvae exposed to normal conditions will be used to infect separate groups of dogs. 4. The proportion of undeveloped larvae to adult worms in each of the groups of dogs will be determined to assess the induction of latency. 5. Hormones, immunosuppressive drugs and toxins which alter the intestinal environment will be examined for their ability to trigger the resumption of larval development.